Combined simulcasting and dedicated services in a wireless communication system

ABSTRACT

An efficient method for providing both dedicated and simulcast services over a common wireless infrastructure is described. The services can be available to a single terminal as well as to a multiplicity of terminals simultaneously. The method uses time division multiplexing and orthogonal frequency division multiple access for simulcasting information and transmitting dedicated message information from a plurality of base stations forming a cellular pattern over the same wireless frequency channel. The method comprises the steps of constructing frames for transmission by the plurality of base stations comprising control information, simulcast information and dedicated message information within predetermined time slots of the frames and allocating the simulcast information and the dedicated message information to time slots of the same frame predetermined by the control information of the frame. The underlying modulation technology used is OFDM and thereby the channel delay-dispersion is minimized.

FIELD OF THE INVENTION

[0001] The invention relates broadly to the field of telecommunicationsand particularly to wireless telecommunication systems. Moreparticularly, it relates to a method for efficiently and flexiblytransmitting both dedicated and simulcast services over wireless systemsby judicious allocation and use of physical communication channels. Thetechnology particularly suitable for providing these services is acombination of Time Division Multiple Access (TDMA) and OrthogonalFrequency Division Multiplexing (OFDM).

BACKGROUND OF THE INVENTION

[0002] Existing wireless systems are configured either for two-waydedicated services or for broadcasting services. The two types ofservices use substantially different technologies and are not amenableto simple, inexpensive and efficient common network implementations.

[0003] Two-way services are dedicated communications between twostations, for example, between a base station and a single mobilesubscriber station. Two-way services between two mobile subscribersmust, by definition, pass through a base station so, for purposes of thepresent discussion, we consider only the transmission between the basestation and a single mobile subscriber station, whether thatcommunication is in one direction or two directions. Two-way servicesmay also include, for example, interactive communication between amobile subscriber and an Internet Web site. For purposes of the presentinvention, two-way communications are a subset of dedicatedcommunications, which can be one-way or two-way but are always addressedto a single user. Current wireless mobile systems are designed to carryprimarily two-way dedicated services.

[0004] Wireless broadcasting systems use fixed and inflexible channelsfor transmitting a single one-directional stream of information from onetransmitter to numerous terminals, as opposed to the use of wirelesssystems. Wireless systems contain common control channels, which areoperated in the broadcast mode, but this type of broadcast channel isconfigured to provide a minimal amount of information and is usedprimarily as a support for dedicated information channels. Short messageservice (SMS) can be carried over broadcast channels, but there aresignificant limitations in the length of SMS driven by physicalconstraints of technologies used for current generation wirelesssystems.

[0005] Apart from broadcast and dedicated transmission modes, certainapplications can take advantage of the simulcast transmission mode. Inthe simulcast mode, a multiplicity of base stations transmits the sameinformation to one or more terminals. Simulcast can be viewed as anaggregation of multiple simultaneous limited-area broadcasts from anumber of base stations, or as an efficient replacement for a large areabroadcast implemented within the wireless network. Simulcasting can beused for paging and high information rate distribution as well as foraudio/video, e.g., digital broadcasting systems. An example of simulcastpaging might be to alert volunteer emergency services personnel (e.g.,volunteer firefighters) of the need to respond, rather than dependingsolely on the old siren. Another example of simulcasting may be anefficient distribution of stock market news by minimizing wirelessresources of the wireless system.

[0006] Both current Time Division Multiple Access (TDMA) and CodeDivision Multiple Access (CDMA) wireless systems have broadcast controlchannels that have severe limitations on their use for informationtransmission. Traditional TDMA systems that could be redesigned based ontraditional technologies to provide high information rate simulcasttransmission will not be efficient. While in the soft-handoff, CDMAsystems operate in a special version of the simulcast mode, whereinformation is sent from many base stations to a single terminal, ratherthan information being sent from a base station (or base stations) to amultiplicity of terminals. However, providing simulcast to mobilestations outside of the soft-handoff region is not feasible with CDMA.As opposed to TDMA and CDMA technologies, the combination of TDMA withOrthogonal Frequency Division Multiplexing (OFDM), which we describe inthis invention, provides a basis for highly efficient simulcasting whichis very well integrated with high-speed dedicated services in a commonwireless system network.

SUMMARY OF THE INVENTION

[0007] The present invention provides a method for efficientlyimplementing both simulcasting and dedicated services within a commonwireless network such that both or either type of service can beprovided simultaneously to different subscriber terminals based on theirindividual needs.

[0008] The provision of wireless Internet service is a rapidly growingbusiness. The Internet provides both two-way (interactive) services suchas Web browsing and broadcast services such as news and entertainmentservices such as live concerts. Third party service providers haveemerged in the market. These third party service providers transform andtranslate Web-based information in order to be able to forwardinformation. This can be in the form of news or sports scores to apaging device, a wireless wireless telephone, a Personal DigitalAssistant (PDA), or a combination of devices. The information may beprovided directly by a provider such as Reuters or ESPN or by a thirdparty that accesses multiple services such as Reuters and ESPN andintegrates the information into a package. Because so many individualssubscribe to such a news or sports service, and because these users arewidely geographically spread within a cellular network, this type ofservice belongs to a class of simulcast services. The information mayalso be provided on an alert basis such as a market watch, where asubscriber wants to be informed if a stock goes above/below a certainamount. This is usually accomplished, however, in the context of one-wayservice such as alerts. In either case, the information received by thesubscriber is based on need and the services to which they havesubscribed. The current way in which this simulcast information isdistributed to the users of a cellular system is by very inefficient useof dedicated channels, such that every subscriber terminal requires onephysical channel to be able to receiver the simulcast.

[0009] Existing wireless technologies provide either broadcastingservices, using one (wireless broadcast) technology, or dedicatedservices using much different (cellular system) technologies.Furthermore, these services are provided to different types ofterminals. A method by which a single network platform, based on thecellular system concept, can provide both types of services is highlydesirable. The present invention provides such a method. Both servicesare provided by the present invention using a combination of wirelessTDMA and OFDM and base station frame synchronization for bothsimulcasting and dedicated services.

[0010] OFDM is currently used in the context of wireless systems andbroadcasting in the European Digital audio Broadcast System (DBS).Broadcasting the information in minimal amounts is supported overcontrol channels in current cellular systems. In contrast, the presentinvention provides for both simulcasting and two-way service and usesthe same cellular network for that purpose. Additionally, in the presentinvention OFDM is not used by itself but in combination with TDMA.

[0011] It is, therefore, an object of the present invention to providemixed services (both dedicated and simulcast) over a common cellularwireless infrastructure. Simulcasting services include, but are notlimited to, high-interest news, stock quotes, sports information andscores. Dedicated service includes service to a single mobilesubscriber, such as phone conversations, as well as Web browsing. Theseservices can be provided to a single terminal (mobile subscriberstation) or simultaneously to a multiplicity of subscribers based ontheir instantaneous needs.

[0012] Further, it an object of the present invention to provide theseservices efficiently. That is, it is an object to minimize powerconsumption, system, base station and subscriber unit complexity andcost. This is accomplished by proposing the methodology that combatschannel dispersion, which is one of the principal degradations thatwireless systems have to combat. With these objects in mind, the presentinvention overcomes significant defects and drawbacks that precludeusing conventional methods for providing such services.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention is best described with reference to the detaileddescription and the following figures, where:

[0014]FIG. 1 illustrates an example of a cellular network that providesboth dedicated channels and simulcast channels to the mobile users.

[0015]FIG. 2 depicts base stations 1 through N each sharing the sameOFDM frequency (carrier), where the carrier is framed in time into timeslots using time-division multiplexing.

[0016]FIG. 3 shows two further concepts attendant to efficientimplementation of the present invention.

[0017] FIGS. 4-6 show the distribution for RMS delay spread illustratingthe significant advantages of the hereby proposed OFDM/TDMA concept forsimulcasting services.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Internet services have emerged that, in some cases, are based onsimulcasting as well as dedicated access. Existing wireless technologiesdo not efficiently support a mixture of simulcasting and dedicatedaccess. The present invention is directed toward a method whereby asystem designed using OFDM and time division multiplexing technologiessupports both simulcasting and, among a group of base stations(proximate located), dedicated access with different informationtransmitted for each subscriber/user.

[0019] Orthogonal Frequency Division Multiplexing (OFDM) is an emergingtechnology for high-speed communications, which we propose to use forwireless communications in a novel and non-obvious manner. OFDM is aspecial form of multi-carrier modulation that uses densely spacedsubcarriers (tones). The information to be communicated is split into Nparallel information streams with each of the information streamsmodulating a different subcarrier of the OFDM carrier. In the moststraightforward version of the system, all base stations use the samecarrier. They share the carrier by time multiplexing (each base stationgetting its turn for transmission during one frame of a predefinedsuperframe time structure). It is, however, also possible to design asystem where each base station may be using only a subset of all thecarriers that comprise the OFDM carrier.

[0020] The subcarriers of a OFDM carrier are chosen to match theproperties of the wireless channel, in particular to be of suchbandwidth to be resistant to delay dispersion experienced bysimulcasting in a cellular system. Analysis and measurements of delayspread impairments show that channel dispersion in mobile channels fordedicated transmission is generally in the 1 to 5 microseconds (μsec)RMS delay spread range. However, simulcast dispersion, where the samesignal is sent from many base stations to a single terminal, is expectedto be two to four times greater. This is illustrated in FIGS. 4-6, whichare obtained for fully synchronized cells using omnidirectionalantennas. FIG. 4 shows the distribution of the RMS delay spread for 380mobile subscriber stations in a system with a large number of basestations. The propagation exponent is 3.7 and there is no shadowing.Four curves are plotted as a function of the cell radius—1, 2, 5 and 10kilometers. It can be observed that the RMS delay spread reaches valuesup to 2, 4, 8 and 18 microseconds, respectively. This is significantlymore than up to 2 microseconds that dedicated channels experience, andmore than can be compensated by terminal equipment. FIG. 5 shows thedistribution of the RMS delay spread for 380 mobile subscriber stationswith the same propagation exponent and shadowing of 8 dB. RMS delayspread range is further extended. FIG. 6 shows the distribution of theRMS delay spread for 760 mobile subscriber stations with a propagationexponent of 3.5 and shadowing of 6 dB. Delay dispersion distributionreaches values as high as 40 microseconds. OFDM modulation is resistantto delay dispersion that is on the order of the guard times betweenmodulation blocks and thereby particularly suitable for combatingsimulcast-induced dispersion illustrated in the previous figures.

[0021] Guard times allocated for dedicated services are appropriate forthose services and guard times between blocks for simulcast services areappropriate for them and different from the guard times allocated fordedicated services. By way of example, using a frame of 20 msec with 20one-msec time slots, time slots used for dedicated services might useOFDM blocks of 150 μsec in length with a 30 μsec guard time. With fivesuch blocks in a one-msec slot, there is a 100 sec time slot guard time.Slots used for simulcasting might operate with four blocks of 150 secwith a 75 sec guard time. This example allocation supports resistance todispersion that is two to three times higher than the dedicated servicesmode. Based on the above example, simulcasting would supportapproximately 80% of the throughput of the dedicated services mode. Thebase stations and network would be effectively and efficiently shared bydedicated services and simulcasting services. The modulation formatswould be the same. Thus, a single terminal (e.g., mobile subscriberstation) could economically receive both simulcast and dedicatedservices.

[0022] OFDM transmission is implemented efficiently using inverse FastFourier Transforms (IFFT) in the transmitter and FFTs in the receiver.Each of the subcarriers or tones is part of the IFFT output. To supportthe sharing of the same OFDM carrier by the multiplicity of basestations, the base stations must be synchronized (which can beaccomplished by the use of a Global Positioning System—GPS) and themobile devices derive their synchronization off the wireless signalssent from the base stations. Since OFDM uses FFT operation, thecomplexity of its implementation is low.

[0023] The present invention uses OFDM for high-speed packet-switched orcircuit-switched wireless information transmission from base stations(on a downlink) in a cellular network on a common frequency (carrier).The transmission signals from all base stations are frame synchronizedin time using GPS signals or by equivalent alternate techniques. Thecommon channel is configured in a TDMA framing manner with a framestructure containing a plurality of time slots, which may be of anyduration in accordance with engineering design considerations (forexample, a 20 msec frame with 20 one-msec time slots).

[0024] Combining OFDM with TDMA in the dedicated mode means that eachframe carries the information for more than one subscriber, where eachsubscriber unit gets its information from one of the slots in the frame(in the dedicated mode). When one slot is dedicated for simulcastingservices, then more than one subscriber unit gets its information from asingle slot. Some time slots are used for simulcasting information frommany base stations (proximately located within the frequency reuse ofthe system). When two or more base stations are simulcasting the (same)signal, they are simulcasting the signal using the same OFDM carrier andthe same TDMA time slot. Thereby, the receiving unit can be tuned to asingle OFDM carrier to receive the simulcast signals.

[0025]FIG. 1 illustrates an example of a cellular network that providesboth dedicated channels and simulcast channels to the mobile users. Basestations 5, 20, 30, 40 and 60 are depicted transmitting dedicatedinformation to mobile terminals 10, 25, 35, 45 and 50 respectively asindicated by the dotted lines connecting the base stations to the mobileterminals. The solid lines indicate a single (one) simulcast informationtransmission from three base stations (20, 40 and 60) to two mobileterminals. Mobile terminal 15 receives the simulcast transmission frombase stations 20, 40 and 60. Mobile terminal 35 receives the simulcasttransmission from base stations 20, 40 and 60. Mobile terminal 65receives a simulcast transmission from base stations 60, 70 and 80 asindicated by the dashed and dotted lines connecting the base stationsand the mobile terminals. Mobile terminal 75 receives the same simulcasttransmission as mobile terminal 65 from the same base stations as mobileterminal 65 (60, 70 and 80). Mobile terminal 75 also receives adedicated transmission from base station 60 as indicated by the dashedline connecting base station 60 and mobile terminal 75. To the mobilesubscriber/user the dedicated transmission and the simulcasttransmission may appear to be simultaneous. They are not actuallysimultaneous but are transmitted as described herein in the same frame.

[0026]FIG. 2 depicts base stations 1 through N each using the same OFDMcarrier, with an assumption that there is only one OFDM carrier. Basestations are mutually synchronized and the signals from each of the basestations are then multiplexed onto the same carrier by using the conceptof time division multiplexing using slots, frames and superframes. Eachtransmitting base station comprises a control processor for use in acombined OFDM and time division multiple access wireless communicationsystem of simulcasting information and transmitting dedicated messageinformation from a plurality of base stations forming a cellular patternover the same wireless frequency channel, including means forconstructing frames for transmission by said plurality of base stationscomprising control information, simulcast information and dedicatedmessage information within predetermined time slots of said frames andmeans for allocating said simulcast information and said dedicatedmessage information to time slots of the same frame predetermined bysaid control information of said frame. The control processor can be aconventional central processing unit, an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or any equivalentmeans for performing the methods disclosed herein.

[0027] As shown in FIG. 2, a few of the time slots in each frame arereserved for control information and purposes, with neighboring/adjacentbase stations using different time slots for this purpose. This is donein order to control the interference. A number of the time slots areused for simulcasting with the same information being transmitted on thesimulcast time slots, same frame and superframe by all base stationsusing a format that mobile subscriber stations can reliably receive. Theremaining time slots are used by all base stations for dedicated orindividual information and can be reused some geographical distanceaway. That is, the time slots used for dedicated transmissions can beand are reused.

[0028] On time slots dedicated to simulcasting, OFDM blocks can bemodified with longer OFDM cyclic extensions to minimize the delaydispersion (which causes multiple copies of the desired signal to bereceived by a mobile subscriber station, coming from several basestations with different propagation delays). Propagation delays may bethe result of signals transmitted by the plurality of proximatelylocated base stations bouncing off a plurality of buildings, mountainsor other objects. Intersymbol interference is reduced by choosing anappropriate OFDM pulse length and cyclic extensions. OFDM system shouldbe adaptive in order to support variable pulse length appropriate forreducing intersymbol interference for the wireless channel at hand.Lengthening the cycling extension reduces the capacity of a system by upto 20-30%, by way of example, but it is necessary to facilitatesimulcast from a number of base stations situated at differentgeographical locations. Because of the redundancy built into an OFDMpulse when used for simulcasting, the original signal can be effectivelyand efficiently received and decoded by the present invention, even whenpropagation delays from different base stations are significantlydifferent.

[0029] There are ways which could further improve the performance of thereception by using multiple antennas, but the concept of multipleantennas does not change the principal idea of the present invention.

[0030] Time slots not used for simulcasting are used for dedicatedservices, where the information transmitted on a single slot by eachbase station is directed to a single user and is different. In the caseof dedicated services, a variety of techniques may be used to minimizethe interference between transmissions of adjacent base stations, suchas intelligent scheduling of transmissions on the available time slots,fixed reuse of resources in time such that adjacent base stations usedifferent time slots or any other equivalent means. The use of OFDM inconjunction with TDMA, along with appropriate network structures,supports both broadcasting/simulcasting and dedicated access on onenetwork with a shared infrastructure.

[0031] Additional options include the provision of simulcasting over alimited area (region). In limited region simulcast, some number of“close-by” base stations will perform a simulcast using a particulartime slot within a given OFDM carrier. This differs from simulcast amongbase stations that are widely separated, which can use the same timeslot at the same carrier frequency for something entirely different,such as dedicated communications. Simulcasting in widely separated basestations results in reuse of the same carrier frequencies and timeslots.

[0032]FIG. 3 shows two further concepts attendant to implementation ofthe present invention. Both frames show the control information in thefirst position. The top frame illustrates the concept of movableboundaries between the simulcast blocks and the dedicated blocks. Forexample, assuming eight time slots per carrier and depending on theneeds of the application, only one time slot may be used forsimulcasting. If the application requires more bandwidth, two or moretime slots may be assigned for simulcasting. This assignment may changeoften and dynamically using the control channel. The remaining timeslots are used for dedicated transmissions.

[0033] The bottom frame suggests the concept of interspersed simulcastblocks and dedicated blocks with appropriate guard times allocated.Interspersing indicates that if more than two time slots are assignedfor simulcasting, they do not have to be adjacent to each other withinthe TDMA frame. For example, time slots one, five and six may be usedfor simulcasting the same information/signal, whereas time slots two,three, four, seven and eight are used for dedicated communications.

[0034] Dedicated service blocks can be managed using any of thecurrently available management techniques or the equivalent. Suchtechniques include fixed assignment, dynamic assignment ornetwork-assisted dynamic packet assignment.

[0035] The system described herein may be implemented in hardware,software or any combination thereof. Further, the present invention maybe implemented using traditional hardware or Application SpecificIntegrated Circuits (ASICs) or Field Programmable Gate Arrays (FPGAs) orany combination thereof. The system may further be implemented as anycombination of hardware, software, ASICs any FPGAs.

[0036] Based on the foregoing, the present invention provides a methodof effectively and efficiently transmitting both dedicated and simulcastservices over a common wireless infrastructure to a single terminalsimultaneously. At the same time, channel dispersion is minimized.

[0037] It should be clear from the foregoing that the objectives of theinvention have been met. While particular embodiments of the presentinvention have been described and illustrated, it should be noted thatthe invention is not limited thereto since modifications may be made bypersons skilled in the art. The present application contemplates any andall modifications that fall within the spirit and scope of theunderlying invention disclosed and claimed herein.

What is claimed is:
 1. A method for use in a time division multipleaccess wireless communication system of simulcasting information andtransmitting dedicated message information from a plurality ofproximately located base stations forming a cellular pattern over thesame wireless frequency channel, the method comprising the steps of:constructing frames for transmission by said plurality of base stationscomprising control information, simulcast information and dedicatedmessage information within predetermined time slots of said frames; andallocating said simulcast information and said dedicated messageinformation to time slots of the same frame predetermined by saidcontrol information of said frame.
 2. The method as recited in claim 1,wherein said control information fills time slots at the beginning ofthe frame and said control information is varied between predeterminedtime slots within said frames such that immediately proximate basestations transmit control information in different predetermined timeslots.
 3. The method as recited in claim 1, wherein said simulcastinformation represents the same information transmitted by saidplurality of base stations and said dedicated message informationcomprises information representing information intended for a singleuser.
 4. The method as recited in claim 1, wherein said allocation ofsaid simulcast information and said dedicated message information tocertain time slots varies over time.
 5. The method as recited in claim1, wherein said time slots of said frame comprise information time andguard time, said information time and said guard time of each time slotvarying in duration over time.
 6. The method as recited in claim 3,wherein said simulcast information fills time slots having an extendedcyclic extension time to mitigate channel dispersion.
 7. The method asrecited in claim 6, wherein said simulcast information time slotextension comprises a guard time approximately twenty-five or higher percent as long as said simulcast information and said dedicatedinformation comprises a guard time of less than twenty-five per cent ofsaid dedicated information.
 8. The method as recited in claim 1, furthercomprising the step of allocating guard time for dedicated informationtime slots differently from allocating guard time for simulcastinformation time slots to mitigate simulcast dispersion and to maximizethroughput of said dedicated message information.
 9. The method asrecited in claim 1, wherein said wireless communication system utilizesorthogonal frequency division multiplexing modulation.
 10. The methodaccording to claim 4, wherein said allocation of simulcast and dedicatedtime slots are interspersed.
 11. The method according to claim 4,wherein said allocation involves setting boundaries between simulcastand dedicated time slots, said boundaries between simulcast anddedicated time slots being movable in accordance with a volume ofsimulcast information and dedicated information.
 12. The methodaccording to claim 11, wherein said boundaries are dynamically movable.13. The method as recited in claim 1, wherein said wirelesscommunication system utilizes a combination of orthogonal frequencydivision multiplexing modulation and time division multiple accessmodulation.
 14. A method for use in a time division multiple accesswireless communication system of simulcasting information andtransmitting dedicated message information from a plurality of basestations forming a cellular pattern over the same wireless frequencychannel, the method comprising the steps of: constructing frames fortransmission by said plurality of base stations comprising controlinformation, simulcast information and dedicated message informationwithin predetermined time slots of said frames; and allocating saidsimulcast information and said dedicated message information to timeslots of the same frame predetermined by said control information ofsaid frame.
 15. The method according to claim 14, wherein said basestations of said cellular pattern are widely separated, resulting in thereuse of said wireless frequency channel and said predetermined timeslots.
 16. A method for use in a combined OFDM and time divisionmultiple access wireless communication system of simulcastinginformation and transmitting dedicated message information from aplurality of base stations forming a cellular pattern over the samewireless frequency channel, the method comprising the steps of:constructing frames for transmission by said plurality of base stationscomprising control information, simulcast information and dedicatedmessage information within predetermined time slots of said frames; andallocating said simulcast information and said dedicated messageinformation to time slots of the same frame predetermined by saidcontrol information of said frame.
 17. An apparatus for use in acombined OFDM and time division multiple access wireless communicationsystem of simulcasting information and transmitting dedicated messageinformation from a plurality of base stations forming a cellular patternover the same wireless frequency channel, comprising a control processorincluding: means for constructing frames for transmission by saidplurality of base stations comprising control information, simulcastinformation and dedicated message information within predetermined timeslots of said frames; and means for allocating said simulcastinformation and said dedicated message information to time slots of thesame frame predetermined by said control information of said frame.